In an image forming apparatus using electrophotographic technology, a one-component development system using only toner as a developer and a two-component development system using both toner and carrier have been known as the methods of developing an electrostatic latent image formed on an image carrying member in the conventional art.
In the one-component development system, toner is generally made to pass through the regulating section formed by a toner carrying member and a regulating plate pressed to the toner carrying member, whereby the toner is charged and is formed into a desired thin toner layer so that an electrostatic latent image is developed. Thus, development is carried out with the toner carrying member placed close to the image carrying member, and this arrangement provides excellent dot reproducibility. And due to a uniform thin toner layer, a uniform image is ensured without irregularity in the image due to a magnetic brush, as is often observed in the two-component development system. This arrangement provides further advantages of simplified apparatus structure, downsized configuration and reduced cost. On the other hand, toner surface is degenerated and charge receptivity is reduced due to heavy stress in the regulating section. Further, the toner regulating member and toner carrying member surface are contaminated by deposition of the toner and external additive agent, and the performance of applying electric charge to toner is reduced. Thus, fogging and contamination inside the apparatus are caused by poorly-charged toner, with the result that the service life of the developing apparatus is reduced.
In the two-component development system, in the meantime, charging is caused by triboelectric charging due to mixture of toner and carrier. This method reduces stress and provides greater resistance to toner deterioration. Further, the carrier as a charge-applying member to toner has a greater surface area, and therefore, this method provides a relatively higher resistance to contamination due to toner and external additive agent, and ensures longer service life.
However, even if the two-component developer is employed, the carrier surface is contaminated by toner and external additive agent all the same. The amount of charge is reduced due to a long-term use, and fogging and toner splashing are caused as a result. The service life cannot be sufficient. There is a need for still longer service life.
A developing apparatus for prolonging the service life of the two-component developer is disclosed in the Unexamined Japanese Patent Application Publication No. S59-100471, wherein the carrier independently or together with toner is supplied little by little, and the deteriorated developer of reduced charging ability is removed accordingly. Thus, the carrier is replaced by new one, and an increase in the proportion of the deteriorated carrier is suppressed. In this apparatus, the reduction in the amount of charging the toner due to carrier deterioration is kept to a predetermined level by replacing the carrier. This arrangement provides advantages in prolonging the service life.
The Unexamined Japanese Patent Application Publication No. 2003-215855 discloses the two-component developer made up of the carrier and toner in which externally added are particles having a charging ability opposite to the polarity of the toner, as well as the development method using this developer. In this development method, the particles having a charging ability opposite to the polarity of the toner are supplied to serve as abrasive powder and spacer particle. It has been demonstrated that deterioration can be effectively suppressed by the effect of removing spent matter on the carrier surface. Further, the cleaning section of the image carrying member is claimed to have the effect of improving the cleaning performance and polishing the image carrying member.
The Unexamined Japanese Patent Application Publication No. H9-185247 discloses the so-called hybrid development system wherein only the toner in the two-component developer is carried on the toner carrying member placed face to face with the image carrying member, thereby developing the electrostatic latent image on the image carrying member. The hybrid development system provides excellent dot reproducibility and image uniformity without image irregularities being caused by a magnetic brush. Due to absence of direct contact between the image carrying member and magnetic brush, movement of the carrier to the image carrying member (carrier consumption) does not occur. This method provides these advantages that cannot be found in the conventional two-component development system. In the hybrid development system, toner is charged by triboelectric charging with the carrier. It is important to maintain excellent carrier charge-applying property for the purpose of stabilizing charging ability of toner, and ensuring a high degree of image quality for a long period of time.
However, one of the common problems with the hybrid development system is that the toner having a high degree of development performance tends to be moved onto the electrostatic latent image carrying member. When continuous printing is performed, the phenomenon of selection occurs wherein the toner of high electrostatic charge performance is accumulated on the toner carrying member, with the result that image density is reduced. Thus, if a toner consumption area and a non-consumption area appear on the toner carrying member, variations occur in the deposition of toner on the toner carrying member and the potential difference of toner. Accordingly, hysteresis occurs at the time of the next development, wherein part of the previous development image appears as a residual image (memory effect). To solve this problem, for example, the Unexamined Japanese Patent Application Publication No. 2002-108104 proposes a developing apparatus wherein the toner on the toner carrying member is recovered by a two-component developer during the non-image formation period among images so as to avoid a residual image.
However, in the Unexamined Japanese Patent Application Publication No. S59-100471, a mechanism for collecting the carrier having been ejected must be installed and a consumable carrier must be used. This has given rise to problems in terms of cost and environment. Further, the required amount of printing must be repeated until the proportion of the new and old carrier is stabilized. Maintenance of the initial characteristics is not necessarily possible. In the Unexamined Japanese Patent Application Publication No. 2003-215855, depending on the image area rate, there is a difference in the amount of consumption of the particles charged opposite to the polarity of the toner. Specifically, when the image area rate is small, there is excessive consumption of the particles charged opposite to the polarity of the toner, the particles being deposited on the non-image section having a greater area. This raises the problem of the suppression effect of carrier deterioration being reduced in the developing apparatus. Further, the hybrid development system disclosed in the Unexamined Japanese Patent Application Publication No. H9-185247 has a problem in that the carrier surface is contaminated by the toner and finishing agent with an increase in the amount of high-volume printing, and the carrier charge-applying property is reduced. In the method disclosed in the Unexamined Japanese Patent Application Publication No. 2002-108104, collection among images becomes difficult when the speed is increased. This requires control in such a way as to increase the space between images (sheets). This raises the problem of productivity being reduced.
An object of the present invention is to provide a developing apparatus using a two-component developer, an image forming apparatus capable of avoiding residual image (memory effect) and stably suppressing the deterioration of the carrier, and a method of forming an image, thereby ensuring long-term formation of a high quality image.